The Harry Potter movies are obviously magical pieces of cinema (a little pun intended). But what makes the movies magical aren't just the elaborate sets, special effects, and ambitious storytelling - it's also the characters.
Stories, no matter how ambitious, simply do not work if they do not contain good characters. In many ways, character design and writing is the most important aspect of storytelling. Even the most well-told story would mean nothing if we didn't care for the characters, their goals, and failures, or their development. The cast of Harry Potter goes through many relationships, some of which are more successful than others.
Ron & Padma
Perhaps the most disastrous relationship throughout the entire series is the one between Ron and Padma Patil. It's not even a relationship - more a "grudging decision to attend the Yule Ball together because Ron is too busy being jealous over Hermione and Viktor."
The entire date is a complete disaster. Ron is gloomy the entire time, and as such, he never even dances with Padma. When she confronts him and directly asks if they're going to dance, he answers with a super cold and decisive "No," prompting Padma to bail. Fans don't blame her in the slightest.
Harry & Parvati
The "relationship" between Harry and Parvati certainly isn't much better. Like Ron and Padma, Parvati wasn't Harry's first choice. He had his eyes on Cho, but when she proved unavailable, he decided to act on Parvati's crush instead. He didn't do it because he liked her - it was more out of total desperation.
This too proved totally disastrous, and Parvati bailed on Harry to go dance with someone else. At least he treated her with a modicum of respect and didn't coldly tell her that they weren't going to dance.
Ron & Lavender
Throughout The Half Blood Prince, Ron becomes the focus of Lavender Brown's infatuation. Much of their relationship is played for laughs, as Ron is completely checked out while Lavender is borderline delirious for him.
Ron can barely stand the sight of her, and he winces in embarrassment when she draws a heart in her breath on the train window. It's probably the worst genuine relationship in the entire movie series, even if it is played largely for comedy. Still, Ron does encourage the relationship, and probably should not have done so.
Harry & Cho
After getting turned down in The Goblet of Fire, Harry lands Cho in The Order of the Phoenix owing to Cedric Diggory's death. It's a bit of an awkward and inappropriate start to the relationship, so points docked there.
They manage to overcome the weird beginnings and become a genuinely loving couple, but the relationship ends in tragedy when Cho betrays Dumbledore's Army to Umbridge (at least in the movies). Though she was under the influence of a potion, Harry can only give Cho the cold shoulder as she attempts to explain herself. Not a great ending.
Viktor & Hermione
The cause of Ron's total consternation, Viktor and Hermione decide to attend the Yule Ball together after Hermione develops a crush on the Quidditch star. By all accounts, the night itself was a success. They both dressed up for the occasion, Viktor showed pride in arriving and attending with Hermione, and he treated her well throughout the night.
Before Ron ruined everything with his jealousy, Hermione was having quite a good time. Their relationship sort of just petered out after the Tournament, so by all accounts it was a success.
Tonks & Remus
One of the most loving relationships in the Harry Potter series is that between Tonks and Remus. The only downside is that it really comes out of nowhere - at least in the movies. By all accounts, they are a loving couple.
They seem to get along well, and they even have a child together. But we don't know much about them. We don't know why they're attracted to each other, or even when they started dating in the first place. It sort of just "happens," and then it ends as quickly as it began. It's a relationship that's sort of just "thrown into" the movies, even though it's a good one.
Ron & Hermione
If nothing else, Ron and Hermione are a controversial couple. At least they get more development than Tonks and Lupin! Some fans don't think they were very compatible and that the relationship was shoehorned into the story.
Others think they make for a perfect "opposites attract," yin-yang sort of relationship. Regardless, they end up together and stay happily married through to the epilogue. However, it's certainly a marriage filled with disagreements and loving bickering, considering their incompatible personalities.
Petunia & Vernon
Petunia and Vernon aren't good people. That much is established. But by all accounts, they make for a good couple. They're certainly old fashioned, as Vernon goes to work and makes the money while Petunia stays home to cook and clean.
But she seems enthusiastic about his career prospects (Chamber of Secrets), and they never tend to fight or disagree. It's also inferred that they've been together a long time - at least the eighteen or so years it took for Harry to grow up and move out. Surely they're doing something right.
Harry & Ginny
Harry and Ginny are another controversial couple, especially in the movies. Many fans found that Daniel Radcliffe and Bonnie Wright shared little, if any, on-screen chemistry, and many criticize the movie for butchering Ginny's character.
Regardless, it's a very successful and loving relationship by all accounts. They never seem to fight, they compliment each other well, and they have a happy family in the epilogue. The problem here arises from acting and on-screen chemistry, not the writing of the characters themselves.
Arthur & Molly
Of course, they have nothing on Arthur and Molly. Easily the series' greatest couple, Arthur and Molly are both great role models for children and a perfect example of a happy, supportive, and loving couple. They certainly have their disagreements, but they always support each other, and viewers get the sense that they would die for each other.
And despite being poor, downtrodden, and rather disrespected in the wizarding world, they have a big, supportive, and happy family. They're great parents, they're great people, and they're certainly a great couple.